Kinda Nitpicking but technically speaking, Android is an Linux Distribution.
An Linux Distribution is an collection of Software that turns the Linux Kernel into an working Operating System.
Android is not GNU/Linux, but it is Linux.
But that beside, the only reason people have an Operating System in the first place is to execute Applications. No user really wants an OS, you need an OS to be able to use Applications on top of that.
Almost all applications that are developed for Smartphones are Android Applications, without Android or at least an Android compatible System, your Smartphone turns mostly into an Feature Phone. That is the reason why Mobian, Ubuntu Touch and so on are all basically only used by the developers who develop them and nobody else.
That SailfishOS was able to gain a userspace outside hackers was because it was Android Compatible, so people could actually execute applications.
But most of these people noticed, that its easier/faster and more convenient to just execute them directly on Android. And those who want an 100% pure FLOSS OS get an more OpenSource OS with AOSP.
I was one of the early adopters of the Jolla Smartphone and used every Version of SailfishOS. IMHO SailfishOS was an insanely clever idea that was executed so bad due to bad management decisions, that its basically dead since.
I just updated my Xperia and bugs i had and reported 10 years ago on my Jolla are still there in the latest release. Third Party support is basically completely dead.
Kudos to everyone still using SailfishOS, but i need a working phone and not a tech gadget toy that sometimes work if i play by its rules and accept what i all can not do with it